It is known that a high level of vibration jeopardizes the mechanical strength of an optical cable, while high temperature, by damaging the optical cladding of the cable, is liable to degrade its transmission performance.
A particularly advantageous application of the invention is for optical cables designed to be disposed the environment of a hot engine, such as an engine in an airplane, or even a rocket launcher, where temperatures can reach 250.degree. C. to 1000.degree. C., and where rms levels of vibration can reach 200 g in a frequency range of 5 Hz to 20,000 Hz.
In a difficult environment, optical cables are generally used in which a single fiber or a bundle of optical fibers is supported mechanically by a sheath made of an organic or plastics material.
However, such protective sheaths have low bending stiffness. Therefore, when an optical information path having high-frequency modes of vibration needs to be designed, a large number of fixing points must be provided along the cable to ensure that lengths between fixing points are short. The natural frequencies of the cable which lie outside the excitation frequencies of the engine for example, guarantee that it is not resonant, and thus guarantee that it is subjected to mechanical stress that is low, thereby ensuring a long life-time for the device. However, due to the large number of fixing points--generally constituted by rings--such a device turns out to be very costly.
In addition, the use of protective sheaths made of plastics material is severely limited at high temperature. Most conventional materials such as Teflon (registered trademark), Kapton, or polymer materials have temperature limits in use of about 260.degree. C. or less.
To solve the problems of mechanical stiffness and ability to withstand high temperatures, it is known to hold optical fibers in metal tubes which are either stiff, or else of the corrugated type, i.e. constituted by a plurality of hinged elements providing them with a certain amount of deformability.
Nevertheless, with such devices, the fiber(s) must be installed inside the metal sheath with a minimum amount of play, so as to reduce vibrations inside the sheath. To further limit said play, a sheath liner made of ceramic wool can also be added during assembly, said liner being taped around the fiber(s), for example.
However, such a sheath liner can be used over short lengths only (30 cm). Furthermore, even with a sheath liner and little play, the resulting support is not, in general, completely satisfactory.